1. Field of the Invention
The present invention relates to a control apparatus of a power steering device for applying a steering assist force by a motor by a steering system of automobile or other vehicle, and more particularly to a control device of a power steering device having functions of monitoring troubles such as intermediate offset or drift among signal troubles of a torque sensor and limiting the assist function.
2. Description of the Related Art
A power steering apparatus for assisting a steering system of an automobile or other vehicle with an assist load by rotating force of a motor is designed to apply an assist load to the steering shaft or rack shaft by transmission mechanism such as gear or belt by way of reduction gears. In a conventional power steering device, in order to generate the assist torque (steering auxiliary torque) accurately, the motor current is controlled by feedback. The feedback control is intended to control the motor applied voltage so as to minimize the difference between the current command value and motor current detection value, and the motor applied voltage is generally controlled by the duty ratio of pulse width modulation (PWM) control.
A general configuration of a power steering device is shown in FIG. 1, in which a shaft 2 of a steering wheel 1 is coupled to a tie rod 6 of turning wheels by way of universal joints 4a and 4b, and a rack-and-pinion mechanism 5. The shaft 2 has a torque sensor 10 for detecting the steering torque of the steering wheel 1, and a motor for assisting the steering force of the steering wheel 1 is coupled to the shaft 2 by way of a clutch 21 and reduction gears 3. A control unit 30 for controlling the power steering device receives an electric power from a battery 14 through an ignition key 11 and a relay 13, and the control unit 30 calculates the steering assist command value I of assist command on the basis of the steering torque T detected by the torque sensor 10 and the vehicle speed V detected by a vehicle speed sensor 12, and controls the current to be supplied to the motor 20 on the basis of the calculated steering assist command value I. The clutch 21 is turned on or off by the control unit 30, and it is turned on (coupled) in an ordinary running state. When the control unit 30 judges the power steering device to be abnormal, or when the power source (voltage Vb) of the battery 14 is turned off by the ignition key 11 and relay 13, the clutch 21 is turned off (disconnected).
The control unit 30 is mainly composed of CPU, and a general function executed by a program in the CPU is shown in FIG. 2.
Explaining the function and operation of the control unit 30, the steering torque T detected and entered by the torque sensor 10 is compensated of phase in a phase compensator 31 in order to heighten the stability of the steering system, and the phase-compensated steering torque TA is inputted to a steering assist command value calculator 32. The vehicle speed V detected by the vehicle speed sensor 12 is also inputted to the steering assist command value calculator 32. The steering assist command value calculator 32 refers to a characteristic map (lookup table) 33 on the basis of the entered steering torque TA and vehicle speed V, and determines the steering assist command value I which is the control target value of the current to be supplied to the motor 20.
The steering assist command value I is inputted to a subtractor 30A, and is also inputted to a differential compensator 34 of feedforward system in order to enhance the response speed, and the deviation (Ixe2x88x92i) determined in the subtractor 30A is inputted to a proportion calculator 35, and the proportion output is inputted to an adder 30B and is also inputted to an integral calculator 36 in order to improve the characteristics of the feedback system. The outputs of the differential compensator 34 and integral calculator 36 are inputted to the adder 30B, and the sum result in the adder 30B, that is, the current command value E is inputted to a motor drive circuit 37 as a motor drive signal. The motor current value xe2x80x9cixe2x80x9d of the motor 20 is detected in a motor current detecting circuit 38, and the motor current detection value xe2x80x9cixe2x80x9d is inputted to the subtractor 30A and is fed back.
Herein, as the conventional torque sensor 10, a type designed to output a main torque signal TM and a sub torque signal TS is used. This is for safety measure in case of failure of the torque sensor 10, and whether trouble or not cannot be judged by one signal output alone. Hitherto, accordingly, monitoring the difference of the main torque signal TM and the sub torque signal TS, and when the difference more than a prescribed value continues for a prescribed time, it is judged to be abnormal, and the assist is cut off.
By using a characteristic map as shown in FIG. 3 having an abrupt rising characteristic, drift of torque sensor or offset voltage in a range not causing serious effect hitherto has come to have a large effect on steering when taking place abruptly while steering, and the assist amount may be excessive depending on the steering status, and an abnormal steering behavior despite the driver""s will may occur, which may lead to a serious accident.
In the conventional trouble detection based on the difference between the main torque signal TM and sub torque signal TS, depending on the change in the drift or offset as shown in FIG. 4, such abnormal behavior may not be detected until the trouble is detected. Further, by setting a severe threshold for detecting trouble, if attempted to detect an intermediate trouble such as offset, considering the adjusting range of the torque sensor before shipping, a severe threshold may lead to wrong detection, and trouble cannot be detected correctly.
A small offset value generated gradually in a long course of time may cause a difference in right and left steering forces, but does not cause uncontrollable behavior of steering. In such a case, not leading to halt of assist, it is unpleasant for the driver, and steering error may occur, and therefore it is necessary to give some warning or limit the assist.
The present invention is devised in the light of such background, and it is hence an object of the invention to present a control apparatus of power steering device having functions of monitoring intermediate troubles such as offset or drift among signal troubles of torque sensor, inhibiting the assist in the event of a relatively large offset, and limiting the assist function when a trouble is detected in the case of a relatively small offset.
The present invention relates to a control apparatus of power steering device for controlling a motor on the basis of a current control value calculated from a steering assist command value calculated by calculating means on the basis of a steering torque generated on a steering shaft, and a current value of a motor for applying a steering assist force to a steering mechanism, and this object is achieved by the configuration in which the difference of a main torque signal and a sub torque signal of a torque sensor for detecting the steering torque is stored preliminarily, and the difference of the main torque signal and the sub torque signal during operation is compared with the stored value, and at least the motor current output is stopped when the state of the difference by the comparison being different from a second prescribed value larger than a first prescribed value continues for a time shorter than a prescribed time.
The object of the present invention is effectively achieved by the configuration in which the steering assist command value is calculated on the basis of the output of a normal mode characteristic map, and when the state of the difference by the comparison being different from the first prescribed value continues for a prescribed time, it is changed over to a torque sensor offset abnormal mode characteristic map, or the current control value is limited according to the vehicle speed, or an intermediate characteristic map is used so as to change without any sense of strangeness when changing over from the normal mode characteristic map to the torque sensor offset abnormal mode characteristic map.